count:统计字段值得数目
查询表中数据总数(文末有sql语句可以直接使用)
SELECT COUNT(*) FROM student ;
COUNT(*) 函数返回表中的记录数(强调表中)
COUNT(*) |
6 |
SELECT COUNT(class) FROM student
WHERE class='3';
COUNT(column_name) 函数返回指定列的值的数目(NULL 不计入):
COUNT(class) |
3 |
SUM:函数返回数值列的总数(总额)。
查询1班的总年龄
SELECT SUM(age) FROM student
WHERE class='1';
SUM(age) |
61 |
AVG: 函数返回数值列的平均值。NULL 值不包括在计算中。
查询3班的平均年龄
SELECT AVG(age) FROM student
WHERE class='3';
AVG(age) |
22.3333 |
MAX:函数返回一列中的最大值。NULL 值不包括在计算中
找出3班年龄最大的
SELECT MAX(age) FROM student
WHERE class='3';
MAX(age) |
24 |
MIN:函数返回一列中的最小值。NULL 值不包括在计算中
找出1班年龄最小的
SELECT MIN(age) FROM student
WHERE class='1';
MIN(age) |
19 |
一个学生信息表结构如下:
id name age teacher class ranking school
1 sunyan 22 a 1 3 x
2 liukan 21 a 3 2 x
3 zhangkai 19 b 1 1 x
4 liuyongting 20 b 1 4 y
5 zhanbin 24 b 3 5 y
6 wangxiao 22 a 3 6 y
2 liukan 21 a 3 2 x
3 zhangkai 19 b 1 1 x
4 liuyongting 20 b 1 4 y
5 zhanbin 24 b 3 5 y
6 wangxiao 22 a 3 6 y
sql语句(拿去直接用)
-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(64) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`age` int(64) DEFAULT NULL,
`teacher` varchar(255) DEFAULT NULL,
`class` varchar(255) DEFAULT NULL,
`ranking` varchar(255) DEFAULT NULL,
`school` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(64) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`age` int(64) DEFAULT NULL,
`teacher` varchar(255) DEFAULT NULL,
`class` varchar(255) DEFAULT NULL,
`ranking` varchar(255) DEFAULT NULL,
`school` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 'sunyan', '22', 'a', '1', '3', 'x');
INSERT INTO `student` VALUES ('2', 'liukan', '21', 'a', '3', '2', 'x');
INSERT INTO `student` VALUES ('3', 'zhangkai', '19', 'b', '1', '1', 'x');
INSERT INTO `student` VALUES ('4', 'liuyongting', '20', 'b', '1', '4', 'y');
INSERT INTO `student` VALUES ('5', 'zhanbin', '24', 'b', '3', '5', 'y');
INSERT INTO `student` VALUES ('6', 'wangxiao', '22', 'a', '3', '6', 'y');
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 'sunyan', '22', 'a', '1', '3', 'x');
INSERT INTO `student` VALUES ('2', 'liukan', '21', 'a', '3', '2', 'x');
INSERT INTO `student` VALUES ('3', 'zhangkai', '19', 'b', '1', '1', 'x');
INSERT INTO `student` VALUES ('4', 'liuyongting', '20', 'b', '1', '4', 'y');
INSERT INTO `student` VALUES ('5', 'zhanbin', '24', 'b', '3', '5', 'y');
INSERT INTO `student` VALUES ('6', 'wangxiao', '22', 'a', '3', '6', 'y');
min()返回最小值(参数可以为字符串)
一个学生信息表结构如下:
id name age teacher class ranking school
1 sunyan 22 a 1 3 x
2 liukan 21 a 3 2 x
3 zhangkai 19 b 1 1 x
4 liuyongting 20 b 1 4 y
5 zhanbin 24 b 3 5 y
6 wangxiao 22 a 3 6 y
2 liukan 21 a 3 2 x
3 zhangkai 19 b 1 1 x
4 liuyongting 20 b 1 4 y
5 zhanbin 24 b 3 5 y
6 wangxiao 22 a 3 6 y
sql语句(拿去直接用)
-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(64) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`age` int(64) DEFAULT NULL,
`teacher` varchar(255) DEFAULT NULL,
`class` varchar(255) DEFAULT NULL,
`ranking` varchar(255) DEFAULT NULL,
`school` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(64) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`age` int(64) DEFAULT NULL,
`teacher` varchar(255) DEFAULT NULL,
`class` varchar(255) DEFAULT NULL,
`ranking` varchar(255) DEFAULT NULL,
`school` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 'sunyan', '22', 'a', '1', '3', 'x');
INSERT INTO `student` VALUES ('2', 'liukan', '21', 'a', '3', '2', 'x');
INSERT INTO `student` VALUES ('3', 'zhangkai', '19', 'b', '1', '1', 'x');
INSERT INTO `student` VALUES ('4', 'liuyongting', '20', 'b', '1', '4', 'y');
INSERT INTO `student` VALUES ('5', 'zhanbin', '24', 'b', '3', '5', 'y');
INSERT INTO `student` VALUES ('6', 'wangxiao', '22', 'a', '3', '6', 'y');
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 'sunyan', '22', 'a', '1', '3', 'x');
INSERT INTO `student` VALUES ('2', 'liukan', '21', 'a', '3', '2', 'x');
INSERT INTO `student` VALUES ('3', 'zhangkai', '19', 'b', '1', '1', 'x');
INSERT INTO `student` VALUES ('4', 'liuyongting', '20', 'b', '1', '4', 'y');
INSERT INTO `student` VALUES ('5', 'zhanbin', '24', 'b', '3', '5', 'y');
INSERT INTO `student` VALUES ('6', 'wangxiao', '22', 'a', '3', '6', 'y');